The invention relates to a jacket ring for the axial-flow low-pressure-turbine region and/or power-turbine.
In low-pressure turbines and power turbines of gas turbines for aircraft, land vehicles and watercraft and also for stationary use, segmented jacket rings (Outer Air Seals, OAS) are arranged above the moving blades and have the following tasks:                screening of the casing of the gas turbine from the high gas temperatures,        providing a run-in lining for the sealing tips at the moving-blade shroud band, and        often also securing the upstream guide-blade stage in the casing by positive locking.        
According to the prior art, at least the last two tasks are assumed by one component, i.e. by the respective jacket ring segment. To fulfill the first task, it is often necessary to arrange additional heat insulation material between the jacket ring and the gas turbine casing enclosing said jacket ring from outside, which increases the costs, the weight and the effort involved during assembly.
Due to the high heat transfer from the run-in lining, usually a honeycomb structure, into the jacket ring segments, which of course also fulfill the guide-blade securing function, the segments become very hot with the following adverse effects:                heating of the guide-blade fastening elements on the casing and of the casing itself by direct, heat-conducting contact,        drop in the strength and in the wear resistance of the heated parts,        dimensional changes and relative movements in the region in which the guide blades are secured, and thus wear and the requirement for large clearances particularly in the axial direction.        
The combination of the two last-mentioned effects has in some cases led to the guide-blade securing function failing as a result of plastic deformations and thus to one or more guide blades being released during operation, thus resulting in considerable damage to or destruction of the turbine region affected.
Such a conventional design of the jacket rings has been disclosed, for example, by DE-C-27 45 130. Here, the jacket rings are designated by “covers (10)” or by “jacket ring covers (10)” and the run-in linings by “sealing surfaces (11)”. The casing (13) of the gas turbine has fastening elements (cylinders 14) which carry both the guide blades (1) and the jacket rings (covers 10). The radially outer, upstream ends of the guide blades (1) enclose widened portions (beads 22) of the fastening elements in a claw-like manner, and the downstream ends of the jacket rings do this in the same way. The radially outer, downstream ends of the guide blades (1) bear radially from inside against the fastening elements (cylinders 14) and are secured in a positive-locking manner by the claw-like, upstream ends of the jacket rings. Due to the construction, therefore, the thermal and mechanical problems already described exist here too.
Against this background, an object of the invention is to design a segmented jacket ring having a sealing carrier function and a guide-blade securing function in such a way that, by thermal relief of the guide-blade fastening points and of the turbine casing, the mechanical loading capacity is increased and thus wear and plastic deformations are greatly reduced or avoided. At the same time, in particular the guide-blade securing function is to be fulfilled with the greatest reliability.
This object is achieved by a plurality of segments which are lined up in the circumferential direction and are arranged radially outside a moving blade ring with a shroud band and inside the casing of the gas turbine, which carry a run-in lining for at least one sealing tip of the shroud band and which hold guide blades in a positive-locking manner at their radially outer downstream end, said guide blades being arranged upstream of the moving blade ring. Each segment (2) comprises a hot-gas-side seal carrier (3), provided with the run-in lining (6), and a casing-side securing element (7) supporting at least one upstream guide blade (15) and extending axially to an extent comparable with the seal carrier (3), in that the seal carrier (3) and the securing element (7) are at a distance from one another over regions of their surfaces which are as large as possible and only have common, heat-conducting contact points (Cl, C2) which are as small as possible, and in that the securing element (7) is axially supported directly on the casing (17) of the gas turbine via a stop part (10).
According to the invention, the seal carrier function on the one hand and the guide-blade securing function on the other hand are in each case fulfilled by a separate component, so that each segment of the jacket ring comprises a seal carrier and a securing element. The seal carrier and the securing element touch one another, but are not firmly connected to one another. The seal carrier and the securing element are designed in relation to one another in such a way that they are at a distance from one another over regions of their surfaces which are as large as possible and only have common, heat-conducting contact points which are as small as possible. The seal carrier more or less absorbs the local temperature of the hot gas during operation, whereas the securing element remains at a markedly lower temperature due to the screening effect of the seal carrier with minimized heat conduction. As a result, the casing-side guide-blade fastening elements also remain markedly cooler during operation and can be loaded mechanically to a considerably higher degree. The temperature reduction continues right into the turbine casing, in the course of which the screening effect of the shell-like securing element also comes to bear. Additional heat-insulation measures or materials are thus as a rule no longer necessary. Owing to the fact that the securing element is axially supported directly on the casing, the guide-blade securing function continues to be reliably fulfilled within the entire operating range irrespective of thermal expansions of the following guide blades.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.